a) Field of the Invention
This invention relates to an array antenna system suitable for mounting on a moving platform such as a ship, and having functions for tracking a target.
b) Description of the Related Art
A stabilized tracking array antenna system tracks a target (e.g. a satellite) while stabilizing an array antenna against the inclination of a moving platform on which it is mounted. Such systems are used, for example, as ship earth stations in the maritime satellite communication managed by INMARSAT (International Maritime Satellite Organization).
An example of a stabilized tracking array antenna system having a construction wherein a parabola antenna is supported by the AZ-EL-XEL mount is shown in FIG. 21, in which a mechanical elevation (EL) axis 12 is pivotally supported by a mechanical azimuth (AZ) axis 14, a mechanical cross-elevation (XEL) axis 10 is pivotally supported by the EL axis 12, and the parabola antenna is pivotally supported by XEL axis 10. Alternatively, the parabola antenna may be supported by an AZ-EL mount, i.e. a simplified mount wherein the mechanical XEL axis 10 is absent.
When using a phased array antenna in place of the parabola antenna, the antenna may be provided with an AZ-EL-virtual XEL mount wherein the AZ axis 14 and EL axis 12 are mechanical, but the XEL axis 10 is virtual. The virtual axis may be implemented by a beam direction control using variable phase-shifters (hereinafter referred as "PSs") about the virtual axis which is perpendicular to the mechanical EL axis 12 and thus the AZ-EL-virtual XEL mount seems similar to the AZ-EL mount in appearance. Systems proposed by the Applicant employ such an AZ-EL-virtual (electronic) XEL mount (see Japanese Patent Application Laid-Open Publication Nos. Hei 4-278703 and 4-315301). The AZ-EL-virtual XEL mount is simpler in its mechanism than that of the AZ-EL-XEL mount, and a tracking error when the elevation of the target is high and the inclination of the moving platform is smaller than that of an AZ-EL mount. The systems proposed by the Applicant, however, all need to input a signal indicating the azimuth of a target (e.g. a satellite) from an external device such as a gyrocompass in order to steer the AZ axis 14 according to the target azimuth.